A SPAD is based on a p-n junction device biased beyond its breakdown region. The high reverse bias voltage generates a sufficient magnitude of electric field such that a single charge carrier introduced into the depletion layer of the device can cause a self-sustaining avalanche via impact ionization. The avalanche is quenched, either actively or passively, to allow the device to be “reset” to detect further photons. The initiating charge carrier can be photo-electrically generated by means of a single incident photon striking the high field region. It is this feature which gives rise to the name ‘Single Photon Avalanche Diode’. This single photon detection mode of operation is often referred to as ‘Geiger Mode’.
SPAD arrays have been used as solid-state detectors in imaging applications where high sensitivity and timing resolution are required. Current state of the art SPAD imaging arrays typically have large (>20 μm) pixel pitches. A smaller pixel pitch facilitates increased resolution for SPAD based 3D Imagers.
It is desirable to provide for smaller SPAD time of flight pixel circuits so as to be able to reduce pixel pitches in SPAD imaging arrays.